Natural gas constitutes an extremely clean burning and efficient source of fuel for many industrial and consumer requirements. However, many sources of natural gas are located remotely from their potential end use sites. Although natural gas is an efficient readily utilizable fuel, it is uneconomic to transport it over great distances because of its gaseous state under ambient conditions. This transportation problem is particularly acute when natural gas must be transported from a remote production site across any substantial body of water before being delivered to its end use site. Exemplary of this is the transportation of natural gas by ship across an ocean. It is uneconomical to transport gaseous natural gas under such conditions. Storage of large quantities of natural gas is also uneconomical when it is in its gaseous state.
However, when natural gas is cooled to liquefaction in order to produce a denser unit of natural gas, it has been found that transportation in a nonpipelined mode can be made more economical. Traditionally, the liquefaction of natural gas for storage and transportation is performed in a system which utilizes a refrigerant cycle or several refrigerant cycles in which the natural gas is cooled and liquefied by heat exchange with such refrigerants. The prior art has taught that natural gas may be precooled against one refrigeration cycle, while being liquefied and subcooled against a subsequent refrigeration cycle which is operated at a lower temperature than the precooled refrigerant cycle.
U.S. Pat. No. 3,763,658 is exemplary of such a natural gas liquefaction cycle. This patent discloses the use of a single component propane refrigeration cycle to precool natural gas and a second multicomponent refrigeration cycle to liquefy and subcool the natural gas. The second low temperature refrigeration cycle is also cooled against the first single component precooled refrigeration cycle.
In U.S. Pat. No. 4,112,700 a liquefaction process is set forth which utilizes a first multicomponent refrigerant comprising 20% ethane and 80% propane and a second multicomponent refrigerant comprising nitrogen, methane, ethane and propane. This patent liquefies the vapor phase first refrigerant against the liquid phase first refrigerant in the same heat exchange which is used to precool the natural gas feed to the process.
U.S. Pat. No. 4,181,174 describes a liquefaction process which utilizes a single component first refrigeration cycle (propane), a multicomponent second refrigeration cycle (methane, ethane, propane and butane) and optionally a third multicomponent refrigeration cycle (methane and butane). Natural gas is cooled and liquefied against the refrigerants in a plate-type heat exchanger.
In U.S. Pat. No. 4,274,849, a process is set forth wherein a gas is liquefied against a main refrigerant of methane, ethane and a substance having a boiling point substantially lower than the methane hydrocarbon. A second auxiliary refrigerating cycle is used to cool the main refrigeration cycle but does not cool the liquefying gas in direct heat exchange. This second refrigeration cycle comprises a two component mixture selected from methane, ethane, propane or butane. Unsaturated or branched forms of the hydrocarbons may also be utilized.
U.S. Pat. No. 4,229,195 discloses a process for the liquefaction of natural gas using a first refrigerant of ethane and propane and a second refrigerant of nitrogen, methane, ethane and propane. The natural gas feed to the process is split into several streams prior to eventual liquefaction.
U.S. Pat. No. 4,339,253 discloses a process for liquefying a gas using two refrigeration cycles in a subcooling heat exchange circuit. Compression requirements are reduced by phase separation and pumping and compressing of the respective liquid and gaseous phases. Each refrigerant can be a multicomponent refrigerant.
As energy requirements become more stringent for the liquefaction of natural gas at its production site in order to render it transportable to an end use site, the liquefaction process and apparatus must necessarily become more efficient in liquefying natural gas. The use of various refrigerant combinations has been attempted by the prior art in order to achieve the goal of liquefaction of natural gas in an efficient manner in a process and system requiring the smallest capital outlay and lowest expenditure of energy possible. In order to maintain natural gas as a competitive fuel, all of these criteria for the processing of natural gas are important. The present invention achieves these objectives of providing an efficient liquefaction scheme which has reduced capital requirements and simplified apparatus and maintenance features.